1 Eco Infra Solution Team 3, SK Engineering&Construction, Seoul 03149, Korea; [email protected] School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 02841, Korea;
[email protected] Technology Research Team, Incheon International Airport Corporation, Incheon 22382, Korea;
[email protected] Advanced Railroad Civil Engineering Division, Korea Railroad Research Institute, Uiwang-si 16105, Korea;
Abstract: Soil conditioning is a key factor in increasing tunnel face stability and extraction efficiencyof excavated soil when excavating tunnels using an earth pressure balance (EPB) shield tunnelboring machine (TBM). Weathered granite soil, which is abundant in the Korean Peninsula (also inJapan, Hong Kong, and Singapore), has different characteristics than sand and clay; it also hasparticle-crushing characteristics. Conditioning agents were mixed with weathered granite soils ofdifferent individual particle-size gradations, and three characteristics (workability, permeability,and compressibility) were evaluated to find an optimal conditioning method. The lower and upperbounds of the water content that are needed for a well-functioning EPB shield TBM were alsoproposed. Through a trial-and-error experimental analysis, it was confirmed that soil conditioningusing foam only was possible when the water content was controlled within the allowable range,that is, between the upper and lower bounds; when water content exceeded the upper bound,soil conditioning with solidification agents was needed along with foam. By taking advantage ofthe particle-crushing characteristics of the weathered granite soil, it was feasible to adopt the EPBshield TBM even when the soil was extremely coarse and cohesionless by conditioning with polymerslurries along with foam. Finally, the application ranges of EPB shield TBM in weathered granite soilwere proposed; the newly proposed ranges are wider and expanded to coarser zones compared withthose proposed so far.
Tunneling work utilizing earth pressure-balanced (EPB) shield tunnel boring machines(TBM) achieve face stability by filling the working chamber with the excavated soil andapplying chamber pressure (face support pressure) toward the tunnel face. Soil condition-ing is needed to increase the tunnel face stability and extraction efficiency of excavatedsoils through screw conveyers. Soil conditioners such as foams and polymers are mostlyinjected into the front of the cutter-head during TBM excavation. They are sometimesinjected into the excavation chamber and the screw conveyor as well when necessary.The appropriate mixing ratio of the soil conditioning agents added to the excavated soilcan be derived through trial and error by evaluating three characteristics of the conditionedsoils, namely workability, compressibility, and permeability. The workability was foundto be reasonable when the slump value of the conditioned soil was between 10 cm and20 cm [1–5]. Furthermore, Wilms [6] suggested that a conditioned soil permeability coeffi-cient less than 1× 10−3 cm/s was required to prevent groundwater inflow into the working
chamber from the tunnel face when tunneling below the groundwater level. Budach [3]proposed a compressibility value of 1.9%/0.5 bar or above for conditioned soil so that theface support pressure will be applied uniformly to the tunnel face in the working chamberand excavated materials will be efficiently extracted from the chamber through screwconveyors. In addition, many other studies were conducted recently, either to find optimalsoil conditioning parameters or to assess the effectiveness of soil conditioning and/or EPBTBM performance [7–9].
Budach and Thewes [1] proposed choosing appropriate soil conditioning agents basedon the particle-size gradation curves in the coarse ground shown in Figure 1; however,we would like to emphasize that the appropriate mixing ratio of the soil conditioningagents might also depend on the characteristics of the soil itself. The characteristics ofweathered granite soil with rock origin of granite or granitic gneiss are different from thoseof sand and clay. The generally understood characteristics of this soil are as follows [10].
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Figure 1. Choice of soil conditioning agent in earth pressure balance (EPB) shield tunnel boring machine (TBM) depending
on particle-size gradation curves in coarse ground soil (Reprinted with permission from ref. [1].).
2. Materials and Methods
2.1. Experimental Equipment and Process
The experimental equipment and testing process are described in [10]. Among them,
the most important equipment for performing conditioning agent mixing experiments us-
ing foam might be the foam generator. For this study, a laboratory-scale foam generator
was produced that could control the foam expansion ratio (FER) and the foam injection
ratio (FIR); see [10] for details. In this experiment, the initial water content of each weath-
ered granite soil sample was set to 10%, considering that the natural water content of sat-
urated soil is more than 11% and less than 21% in most cases. Then, after foam was gen-
erated for each FIR, additional water was added up to the designated water content, and
these were mixed together with an agitator. Immediately after mixing, slump, permeabil-
ity, and compressibility were tested. In this permeability test, attention should be paid so
that the filters installed both at the top and bottom of the sample are fully saturated before
the test. Furthermore, since the foam itself may be drained for a long period of time, long-
term changes in the permeability coefficient were observed over a longer period. For the
detailed experimental methods of these three tests, please refer to [10].
2.2. Weathered Granite Soils and Conditioning Agents Used in Experiments
2.2.1. Weathered Granite Soils
Five weathered granite soil samples were prepared, ranging from relatively fine to
coarse; the particle-size gradation curves for these five samples are shown in Figure 2
(solid lines); properties of these five samples are shown in Table 1. As shown in Table 1,
the percentage of fine particles passing through #200 sieve changed from 17.1% (for Soil
1) to nil (for Soil 5). After the experiments, sieve analysis was conducted again for the
oven-dried samples to obtain the particle-size gradation curves in order to find the parti-
cle-crushing effect after soil conditioning; these are represented by the dotted lines in Fig-
ure 2. The particle-crushing effect is predominantly dependent upon the particle size of
the soils. Figure 2 shows that the percentage of fine particles passing through #200 sieve
Figure 1. Choice of soil conditioning agent in earth pressure balance (EPB) shield tunnel boring machine (TBM) dependingon particle-size gradation curves in coarse ground soil (Reprinted with permission from ref. [1].).
First, the particle-size gradation of the weathered granite soil has a wide spectrumof coarse to fine particles depending on the rock origin and weathering process; the char-acteristics of these soils are unique, neither sand nor clay. Second, the particle-crushingcharacteristics of the weathered granite soil appear to be more dominant than those ofsand [11–13]. As such, the particle-size gradation curve of weathered granite soil changesafter soil conditioning; in particular, the soil particle percentage that passes through a #200sieve increases after soil conditioning. Third, weathered granite soil has a complicatedrelationship with water that depends on the water content. The natural water content of thesaturated granite soils (below the groundwater table) on the Korean peninsula are withinthe range of 11–21% in most cases [14]. Particle-crushing capacity increases if water isadded to granite soils, increasing the compressibility and decreasing the shear strength [12].Kim et al. [10] performed a preliminary study on choosing a soil conditioning agent using
Appl. Sci. 2021, 11, 2995 3 of 15
two types of weathered granite soils that were within the previously proposed applicationrange of EPB shield TBM shown in Figure 1 proposed by [1]; the authors found that soilscan be well-conditioned with only foam, that is, without polymers or fines, if the watercontent of the excavated soil is at optimal values. This research extended that preliminaryresearch by [10] through the following comprehensive studies:
(1) More comprehensive ranges were studied of weathered granite soils having differentparticle-size gradation curves and percentages of fine particles (less than #200 sievesize) ranging from 0% (extremely cohesionless soil) to 17.1%.
(2) Upper and lower bounds of water content, as well as optimum contents dependentupon fine particle-size contents, were tested. If the water content of the excavated soilwas more than the upper bound value, the slump value might be larger than 20 cm,which is too fluidic; then, extracting the excavated soil through a screw conveyor maynot feasible. If the water content is less than the lower bound value, then it is notfeasible to achieve the slump value of 10 cm without adding water.
(3) Finally, application ranges of EPM shield TBM and choice of conditioning agents weredetermined for weathered granite soils; the ranges depend on the soil’s water content.The ranges proposed in this study may differ from those proposed by [1], and one ofthe primary reasons for this difference might be the particle-crushing characteristicsof weathered granite soil.
2. Materials and Methods2.1. Experimental Equipment and Process
The experimental equipment and testing process are described in [10]. Among them,the most important equipment for performing conditioning agent mixing experimentsusing foam might be the foam generator. For this study, a laboratory-scale foam generatorwas produced that could control the foam expansion ratio (FER) and the foam injection ratio(FIR); see [10] for details. In this experiment, the initial water content of each weatheredgranite soil sample was set to 10%, considering that the natural water content of saturatedsoil is more than 11% and less than 21% in most cases. Then, after foam was generatedfor each FIR, additional water was added up to the designated water content, and thesewere mixed together with an agitator. Immediately after mixing, slump, permeability,and compressibility were tested. In this permeability test, attention should be paid so thatthe filters installed both at the top and bottom of the sample are fully saturated before thetest. Furthermore, since the foam itself may be drained for a long period of time, long-termchanges in the permeability coefficient were observed over a longer period. For the detailedexperimental methods of these three tests, please refer to [10].
2.2. Weathered Granite Soils and Conditioning Agents Used in Experiments2.2.1. Weathered Granite Soils
Five weathered granite soil samples were prepared, ranging from relatively fine tocoarse; the particle-size gradation curves for these five samples are shown in Figure 2(solid lines); properties of these five samples are shown in Table 1. As shown in Table 1,the percentage of fine particles passing through #200 sieve changed from 17.1% (for Soil1) to nil (for Soil 5). After the experiments, sieve analysis was conducted again for theoven-dried samples to obtain the particle-size gradation curves in order to find the particle-crushing effect after soil conditioning; these are represented by the dotted lines in Figure 2.The particle-crushing effect is predominantly dependent upon the particle size of the soils.Figure 2 shows that the percentage of fine particles passing through #200 sieve graduallyincreases with the increase of soil particle sizes (except for Soil 5, which is classified asgravel): increases of 3.7% for Soil 1, 4% for Soil 2, 4.7% for Soil 3, and as much as 5% forSoil 4. As can be seen from regions I, II, and III of Figure 1, fine particle contents in thegradation curve of soils are the most crucial factor when determining the conditioningagent in the EPB Shield TBM method. For this reason, the particle-crushing effect of the
Appl. Sci. 2021, 11, 2995 4 of 15
weathered granite soil must be taken into consideration when assessing the conditioningagent. This will be further discussed in Section 3.
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gradually increases with the increase of soil particle sizes (except for Soil 5, which is clas-
sified as gravel): increases of 3.7% for Soil 1, 4% for Soil 2, 4.7% for Soil 3, and as much as
5% for Soil 4. As can be seen from regions I, II, and III of Figure 1, fine particle contents in
the gradation curve of soils are the most crucial factor when determining the conditioning
agent in the EPB Shield TBM method. For this reason, the particle-crushing effect of the
weathered granite soil must be taken into consideration when assessing the conditioning
agent. This will be further discussed in Section 3.
Figure 2. Particle-size gradation curves for the five weathered granite soils.
Table 1. Physical properties of the five weathered granite soils.
Soil 1 Soil 2 Soil 3 Soil 4 Soil 5
Percent passing through a #200 sieve (%) 17.1 11.8 6.5 1.2 0
The conditioning agents used for the EPB shield TBM include water, foam, polymer,solidification agent (water-absorbing polymer), bentonite, and anti-clay polymer. The con-ditioning agents used in this experiment and their physical properties are summarizedin Table 2. Foam added to the excavated soil plays roles in increasing compressibility,reducing the internal friction angle, maintaining the uniform distribution of the chamberpressure, and reducing the permeability [15]. In these experiments, an FER of 10 was used,which is a normal practice in situ [10].
Polymers are used in coarse soil with high permeability and/or when tunnelingwork is going on below the groundwater level. In other words, the use of polymers isrecommended when mixing with foam alone is not sufficient. Soil-structuring polymerused in porous ground forms networks through chained structures to increase the cohesionof the excavated soil and reduce the internal friction angle [15,16]. As a result, the soil-structuring polymer helps the conditioned soil to behave in a pseudo-plastic state and
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decreases the soil’s permeability. The polymer slurry used in these experiments was anemulsion type with high viscosity; it was diluted to 0.3% concentration before it was addedto the soil samples. Polymer slurry can be injected either at the front of the cutter head orwithin the chamber, with or without other conditioning agents. In contrast, the polymerused as a solidification agent (known as water-absorbing polymers or thickeners) is notdiluted but is directly added to the working chamber or the screw conveyor as is. By theway, for the definition of parameters to describe the quantity and quality of the conditioningagents such FER, FIR, and polymer injection ratio (PIR), please refer to [10].
Table 2. Properties of conditioning agents used in the experiment.
Category Foam Polymer 1 Polymer 2
Product name MAK Foam SUPER MUD (emusion) MAK SOL-L (liquid)
Type Foam Polymer slurry Water-absorbing polymer(solidification agent)
3. Results and Discussion3.1. Ranges of Water Content for Workability Requirement
As mentioned in Section 1, in order to provide the desired workability for the EPBshield TBM operation, slump values of the conditioned soils should be between 10 cm and20 cm. Kim et al. [10] mentioned that controlling the water content inside the workingchamber is as important as controlling the FIR.
Figure 3 shows typical measured slump values with different water contents and FIRsfor the four soil samples (Soil 1 to 4) in Figure 2 and Table 1; pictures taken after slumptests are shown in Figure 4. The FIRs varied from small (22%) to large (88%); the normalrange is 30% to 80%. Referencing the experimental results of Figure 3a, slump tests werenot conducted thereafter if we expected the slump values to either exceed 20 cm or bemuch less 10 cm (please see missing data in Figure 3b,d).
For each sample, the water content that reached the slump value of as small as 10 cmwas obtained in spite of adding foam with the maximum FIR of 88% was designated thelower bound of water content. On the contrary, the water content that reached the slumpvalue as large as 20 cm was obtained in spite of adding foam with the minimum FIR of22% was designated the upper bound of water content. These results are summarized inFigure 5; the figure shows the upper and lower bounds of water content for slump values of10–20 cm as a function of the percentage of fine particles in excavated soil passed througha #200 sieve (from 1.2% to 17.1%).
As shown in Figure 5, the water content to reach the slump values of 10 cm to 20 cmincreased as the fine particle content increased. Curve A in Figure 5, indicating the medianbetween the lower and upper bounds, might represent the optimum water contents asa function of the fine particle content passing through the #200 sieve. From this figure,the following strategy for choosing a soil conditioning agent can be proposed.
(1) If the water content of the excavated soil is within the allowable range proposed inFigure 5, the workability requirement is satisfied with slump values between 10 cmand 20 cm. In this case, we have to check whether the conditioned soils also satisfythe permeability and compressibility requirements; if these are satisfied, it is sufficientto add only foam as the soil conditioning agent. This will be further discussed inSection 3.2
(2) If the water content of the excavated soil is below the lower bound values, theworkability requirement for slump value greater than 10 cm will not be satisfied;water must be added to the working chamber to reach the optimum values shown oncurve A.
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(3) If the water content is above the upper bound shown in Figure 5, the workabilityrequirement for slump value less than 20 cm will not be satisfied, requiring addedsolidification agents in addition to foams. This will be discussed in Section 3.3.1.
(4) Finally, if the weathered granite soil is extremely coarse and cohesionless with fineparticle percentage less than 1.2% (Soil 5 in Table 1), the particle-size gradation curveshown in Figure 2 is located outside (to the right) of the applicable ranges proposedby [1] (see Figure 1). To take advantage of the particle-crushing characteristics ofweathered granite soil, it may be necessary to check the feasibility of applying theEPB shield TBM in this outside region also. Additional conditioning agents such aspolymer slurries and/or solidification agents (water-absorbing polymers) may berequired in this region in addition to foams. This will be discussed in Section 3.3.2.
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the definition of parameters to describe the quantity and quality of the conditioning
agents such FER, FIR, and polymer injection ratio (PIR), please refer to [10].
Table 2. Properties of conditioning agents used in the experiment.
Category Foam Polymer 1 Polymer 2
Product name MAK Foam SUPER MUD (emusion) MAK SOL-L (liquid)
Type Foam Polymer slurry Water-absorbing polymer (solidification agent)
3.1. Ranges of Water Content for Workability Requirement
As mentioned in Section 1, in order to provide the desired workability for the EPB
shield TBM operation, slump values of the conditioned soils should be between 10 cm and
20 cm. Kim et al. [10] mentioned that controlling the water content inside the working
chamber is as important as controlling the FIR.
Figure 3 shows typical measured slump values with different water contents and
FIRs for the four soil samples (Soil 1 to 4) in Figure 2 and Table 1; pictures taken after
slump tests are shown in Figure 4. The FIRs varied from small (22%) to large (88%); the
normal range is 30% to 80%. Referencing the experimental results of Figure 3a, slump tests
were not conducted thereafter if we expected the slump values to either exceed 20 cm or
be much less 10 cm (please see missing data in Figure 3b,d).
(a) (b)
(c) (d)
Figure 3. Slump values with variation in the water content and foam injection ratio (FIR) [10]: (a)
Soil 1, (b) Soil 2, (c) Soil 3, (d) Soil 4.
0
50
100
150
200
250
300
0 30 60 90
Slu
mp
va
lue
(mm
)
FIR (%)
w = 35 %
w = 30 %
w = 25 %
0
50
100
150
200
250
300
0 30 60 90
Slu
mp
valu
e (m
m)
FIR (%)
w = 27.5 %
w = 22.5 %
w = 17.5 %
0
50
100
150
200
250
300
0 30 60 90
Slu
mp
va
lue
(mm
)
FIR (%)
w = 23 %
w = 18 %
w = 13 %
0
50
100
150
200
250
300
0 30 60 90
Slu
mp
valu
e (m
m)
FIR (%)
w = 20 %
w = 15 %
w = 10 %
Figure 3. Slump values with variation in the water content and foam injection ratio (FIR) [10]: (a)Soil 1, (b) Soil 2, (c) Soil 3, (d) Soil 4.
3.2. Optimal Mixing Ratio of Conditioning Agents: With Foam Only
As mentioned in the previous section, if the water content in the working chamberis controlled within the allowable ranges proposed in Figure 5, the workability of theexcavated soils will be satisfied by conditioning using foam only. Permeability and com-pressibility tests were performed on the soil samples conditioned with foam (Soil 1 throughSoil 4), in which the water contents were controlled within allowable ranges. Since Kimet al. [10] revealed in their preliminary study that soils conditioned with foam satisfy allthe requirements when FIR can be controlled between 22% and 67% in the optimum watercontents shown in curve A in Figure 5, in this study, all the tests were done under the same
Appl. Sci. 2021, 11, 2995 7 of 15
conditions. Experimental results are summarized in Table 3; the table shows that as theFIR increased, the permeability coefficient decreased while the compressibility increasedas well. Table 3 shows that all the conditioned soil samples met the three requirements:slump value between 10 cm and 20 cm, permeability coefficient less than 1 × 10−3 cm/s,and compressibility more than 1.9%/0.5 bar.
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FIR 22% FIR 45% FIR 67%
(a)
FIR 22% FIR 45% FIR 67%
(b)
FIR 22% FIR 45% FIR 67%
(c)
FIR 22% FIR 45% FIR 67%
(d)
Figure 4. Slump test results with variation in the water content and FIR [10]: (a) Soil 1 (water con-
The measured permeability coefficient summarized in Table 3 is the initial measure-ment value; however, at the job site, it would be necessary to maintain this low coefficientfor at least 90 min to provide good workability during ring assembly and other processes [3].Therefore, the change of permeability coefficients with elapse of time was monitored upto 10 h; monitored results are shown in Figure 6. The figure shows that even though thepermeability coefficients tended to increase with time, the final values were still far below1 × 10−3 cm/s.
In summary, in weathered granite soil with fine particles at least 1.2%, the excavatedsoils can be conditioned with foam alone as long as the water contents are controlledwithin the allowable values (and preferably the optimal values) shown in Figure 5 and theconditioned FIR is controlled within the range of 22% to 67%.
3.3. Optimal Mixing Ratio of Conditioning Agents: With Foam and Polymer3.3.1. Conditioning with Foam and Water-Absorbing Polymer
As mentioned in Section 3.1, if the water content in excavated soil exceeds the upperbound shown in Figure 5, the workability requirement of slump value less than 20 cmwill not be satisfied; solidification agents need to be added in addition to foam. Therefore,a series of tests were performed on excavated soils in which water contents exceeded the
Appl. Sci. 2021, 11, 2995 9 of 15
upper bound shown in Figure 5. In these tests, all the soil samples were conditioned withfoam added at a minimum FIR of 22% and with and without adding the water-absorbingpolymer; typical results are shown in Figure 7.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 15
creased as well. Table 3 shows that all the conditioned soil samples met the three require-
ments: slump value between 10 cm and 20 cm, permeability coefficient less than 1 × 10−3
cm/s, and compressibility more than 1.9%/0.5 bar.
The measured permeability coefficient summarized in Table 3 is the initial measure-
ment value; however, at the job site, it would be necessary to maintain this low coefficient
for at least 90 min to provide good workability during ring assembly and other processes
[3]. Therefore, the change of permeability coefficients with elapse of time was monitored
up to 10 h; monitored results are shown in Figure 6. The figure shows that even though
the permeability coefficients tended to increase with time, the final values were still far
below 1 × 10−3 cm/s.
In summary, in weathered granite soil with fine particles at least 1.2%, the excavated
soils can be conditioned with foam alone as long as the water contents are controlled
within the allowable values (and preferably the optimal values) shown in Figure 5 and
the conditioned FIR is controlled within the range of 22% to 67%.
Table 3. Properties of conditioned soils depending on foam mix ratios.
Soil FER FIR (%) w (%) Slump (cm) Permeability Coefficient (cm/s) Compressibility (%/0.5 Bar)
Weathered Granite Soil 1
(#200 finer = 17.1%)
0 0 10 None 2.93 × 10−4 0.48
10 22 30 11.8 - 3.18
10 45 30 13 - 4.36
10 67 30 16.5 - 5.02
Weathered Granite Soil 2
(#200 finer = 11.8%)
0 0 10 None 4.46 × 10−4 0.38
10 22 22.5 11 - 3.02
10 45 22.5 14.5 1.98 × 10−6 4.16
10 67 22.5 19 - 4.98
Weathered Granite Soil 3
(#200 finer = 6.5%)
0 0 10 None 9.13 × 10−4 0.29
10 22 18 11.5 - 2.90
10 45 18 15 2.73 × 10−6 4.12
10 67 18 19.5 - 4.86
Weathered Granite Soil 4
(#200 finer = 1.2%)
0 0 10 None 1.40 × 10−3 0.24
10 22 15 12 2.16 × 10−6 2.74
10 45 15 16 1.62 × 10−6 4.04
10 67 15 19 1.17 × 10−6 4.84
Figure 6. Permeability coefficient changes of conditioned soils with elapse of time (Soil 2–4).
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
0 100 200 300 400 500 600
Per
mea
bil
ity C
oef
fici
ent
(cm
/s)
Time (min)
Soil 2 (FIR = 45 %) Soil 3 (FIR = 45 %)
Soil 4 (FIR = 22 %) Soil 4 (FIR = 45 %)
Soil 4 (FIR = 67 %)Soil 4 (un-conditioned)
Soil 3 (un-conditioned)Soil 2 (un-conditioned)
1×10-2
1×10-3
1×10-4
1×10-5
1×10-6
1×10-7
Figure 6. Permeability coefficient changes of conditioned soils with elapse of time (Soil 2–4).
Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 15
3.3. Optimal Mixing Ratio of Conditioning Agents: With Foam and Polymer
3.3.1. Conditioning with Foam and Water-Absorbing Polymer
As mentioned in Section 3.1, if the water content in excavated soil exceeds the upper
bound shown in Figure 5, the workability requirement of slump value less than 20 cm will
not be satisfied; solidification agents need to be added in addition to foam. Therefore, a
series of tests were performed on excavated soils in which water contents exceeded the
upper bound shown in Figure 5. In these tests, all the soil samples were conditioned with
foam added at a minimum FIR of 22% and with and without adding the water-absorbing
polymer; typical results are shown in Figure 7.
(a) (b)
Figure 7. Samples conditioned with and without adding water-absorbing polymer in addition to
foam: (a) foam only, (b) foam and water-absorbing polymer.
When only foam was added, the soil samples were too fluidic, whereas when both
foam and water-absorbing polymer are added together, the conditioned soil appears to
be in a plastic state. Experimental results are summarized in Table 4; the table shows that
if only foam was added as a conditioning agent, the soil samples appeared to be too fluidic
to measure the slump value. However, when water-absorbing polymer (not diluted) was
added along with foam, we were able to match the slump value requirement of 10 cm to
20 cm; please see Figure 8 showing pictures of soil samples taken after slump tests. The
“N/107.8 N” in Table 4 indicates the amount (in Newtons) of water-absorbing polymer
injected into the conditioned soil with the amount of 107.8 N.
Table 4. Properties of conditioned soils depending on added amount of water-absorbing polymer.
Figure 7. Samples conditioned with and without adding water-absorbing polymer in addition tofoam: (a) foam only, (b) foam and water-absorbing polymer.
When only foam was added, the soil samples were too fluidic, whereas when bothfoam and water-absorbing polymer are added together, the conditioned soil appears to bein a plastic state. Experimental results are summarized in Table 4; the table shows that ifonly foam was added as a conditioning agent, the soil samples appeared to be too fluidicto measure the slump value. However, when water-absorbing polymer (not diluted) wasadded along with foam, we were able to match the slump value requirement of 10 cmto 20 cm; please see Figure 8 showing pictures of soil samples taken after slump tests.The “N/107.8 N” in Table 4 indicates the amount (in Newtons) of water-absorbing polymerinjected into the conditioned soil with the amount of 107.8 N.
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Table 4. Properties of conditioned soils depending on added amount of water-absorbing polymer.
Figure 8. Soil samples after slump tests at varying amounts of added water-absorbing polymers.
Even though it was observed that the compressibility decreased (slightly reduced by
adding the water-absorbing polymers by up to 26%), these values were still larger than
the required 1.9%/0.5 bar. In summary, excavated soils with water contents above the up-
per bound values proposed in Figure 5 can meet the workability, permeability, and com-
pressibility requirements if they are conditioned with foam added at approximately 22%
FIR and around 0.39 N of water-absorbing polymer per 107.8 N of excavated soil.
3.3.2. Conditioning with Foam and Multiple Polymer
If the weathered granite soil is extremely cohesionless (coarse) with fine particle per-
centage less than 1.2% (Soil 5 in Table 1), the particle-size gradation curve shown in Figure
2 is located outside (to the right) of the applicable ranges proposed by [1], as mentioned
in Section 3.1. In other words, according to their proposal, the EPB shield TBM cannot be
adopted in this region. However, to take advantage of the particle-crushing characteristics
of weathered granite soil, the feasibility should be checked of applying the EPB shield
TBM in this region also. However, the region may need additional conditioning agents
such as water-absorbing polymers or polymer slurries in addition to foam.
A series of tests were performed on Soil 5 shown in Figure 2 and Table 1, in which
the fine particle content was zero. As the conditioning agents, foam was added at FIRs
ranging from 22% to 67% with and without polymer slurry added at PIRs ranging from
8% to 30%. The optimal water content was obtained following trial and error; to satisfy
the workability requirement of slump value between 10 and 20 cm, 10% water content
was used. Experimental results are summarized in Table 5, and pictures of the slump test
results are shown in Figure 9. Table 5 shows that even though the slump value and com-
pressibility requirements were satisfied by adding foam only at FIR of 45%, it was not
possible to meet the permeability requirement (slump requirement was not satisfied with
FIR = 67%; compressibility requirement was not satisfied with FIR = 22%). Soil 5 was too
porous to reduce the permeability coefficient to the thrust value of 1 × 10−3 cm/s with only
foam.
Table 5. Properties of conditioned Soil 5 depending on the mixing ratios of conditioning agents.
w (%) FIR (%) PIR (%) Slump (cm) Compressibility
(%/0.5 Bar)
Permeability Coefficient
(cm/s)
Foam conditioning
10 22 0 12 1.22 -
10 45 0 16 2.67 1.17 × 10−3
10 67 0 22 3.90 -
Foam and Polymer
slurry conditioning
10 22 8 18 2.07 1.67 × 10−4
10 22 15 16 1.72 1.87 × 10−4
10 22 30 - - -
Figure 8. Soil samples after slump tests at varying amounts of added water-absorbing polymers.
Even though it was observed that the compressibility decreased (slightly reducedby adding the water-absorbing polymers by up to 26%), these values were still largerthan the required 1.9%/0.5 bar. In summary, excavated soils with water contents abovethe upper bound values proposed in Figure 5 can meet the workability, permeability,and compressibility requirements if they are conditioned with foam added at approximately22% FIR and around 0.39 N of water-absorbing polymer per 107.8 N of excavated soil.
3.3.2. Conditioning with Foam and Multiple Polymer
If the weathered granite soil is extremely cohesionless (coarse) with fine particlepercentage less than 1.2% (Soil 5 in Table 1), the particle-size gradation curve shownin Figure 2 is located outside (to the right) of the applicable ranges proposed by [1],as mentioned in Section 3.1. In other words, according to their proposal, the EPB shieldTBM cannot be adopted in this region. However, to take advantage of the particle-crushingcharacteristics of weathered granite soil, the feasibility should be checked of applying theEPB shield TBM in this region also. However, the region may need additional conditioningagents such as water-absorbing polymers or polymer slurries in addition to foam.
A series of tests were performed on Soil 5 shown in Figure 2 and Table 1, in whichthe fine particle content was zero. As the conditioning agents, foam was added at FIRsranging from 22% to 67% with and without polymer slurry added at PIRs ranging from8% to 30%. The optimal water content was obtained following trial and error; to satisfythe workability requirement of slump value between 10 and 20 cm, 10% water content
Appl. Sci. 2021, 11, 2995 11 of 15
was used. Experimental results are summarized in Table 5, and pictures of the slumptest results are shown in Figure 9. Table 5 shows that even though the slump value andcompressibility requirements were satisfied by adding foam only at FIR of 45%, it was notpossible to meet the permeability requirement (slump requirement was not satisfied withFIR = 67%; compressibility requirement was not satisfied with FIR = 22%). Soil 5 was tooporous to reduce the permeability coefficient to the thrust value of 1 × 10−3 cm/s withonly foam.
Table 5. Properties of conditioned Soil 5 depending on the mixing ratios of conditioning agents.
w (%) FIR (%) PIR (%) Slump (cm) Compressibility(%/0.5 Bar)
PermeabilityCoefficient (cm/s)
Foamconditioning
10 22 0 12 1.22 -10 45 0 16 2.67 1.17 × 10−3
10 67 0 22 3.90 -
Foam andPolymer slurryconditioning
10 22 8 18 2.07 1.67 × 10−4
10 22 15 16 1.72 1.87 × 10−4
10 22 30 - - -Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 15
(a) (b)
Figure 9. Slump test results depending on added conditioning agents: (a) Foam conditioning only, (b) Foam and polymer
slurry conditioning.
As can be observed from the appearance of the conditioned soil in Figure 9a, foam
and soil did not interact as a fully mixed material. The foam segregated from the soil,
moved upward when tamped with a tamping rod to prepare for the slump tests, and fi-
nally flowed out from the soil samples as shown in Figure 9a. This segregation behavior
between foam and excavated soil might reduce compressibility and/or impede reducing
permeability.
Table 5 also presents the experimental results when the soil samples were condi-
tioned with both foam and polymer slurry. Since the compressibility requirement can be
easily met by adding polymer slurries, the minimum FIR was chosen to be 22%. The PIR
of the polymer slurry (concentration of 0.3%) was varied from 8% to 30%. The table shows
that the slump value requirement was satisfied when the PIR was either 8% or 15%. How-
ever, foam and polymer outflow were observed during the slump test, as can be seen in
Figure 9b when the PIR was at a higher value of 15%. The segregation behavior also re-
duces the compressibility of the conditioned soil. At PIR of 30%, the foam and the polymer
experienced more serious material segregation during the slump test, and the specimen
collapsed (see Figure 9b). This means that the plastic behavior of excavated soils is dis-
rupted when excess polymer slurry is used.
In contrast, when the polymer slurry was added at PIR of 8%, the compressibility
increased; this was because the highly viscous polymer slurry trapped the foam so that it
did not flow out from the soil sample. In other words, the polymer slurry changed the
rheology of the excavated soils, preventing the segregation behavior [16]. Table 5 and Fig-
ure 10 show that the soils conditioned by adding foam at FIR = 22% together with polymer
slurry at PIR = 8% or 15% met the required permeability coefficient of 1 × 10−3 cm/s.
Figure 10. Permeability coefficient changes of the conditioned Soil 5 with elapse of time.
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
0 100 200 300 400 500 600
Per
mea
bil
ity C
oef
fici
ent
(cm
/s)
Time (min)
Un-conditioned FIR 45 %
FIR 22 % + PIR 8 % FIR 22 % + PIR 15 %
1×10-2
1×10-3
1×10-4
1×10-5
1×10-1
Figure 9. Slump test results depending on added conditioning agents: (a) Foam conditioning only, (b) Foam and polymerslurry conditioning.
As can be observed from the appearance of the conditioned soil in Figure 9a, foam andsoil did not interact as a fully mixed material. The foam segregated from the soil, moved up-ward when tamped with a tamping rod to prepare for the slump tests, and finally flowedout from the soil samples as shown in Figure 9a. This segregation behavior between foamand excavated soil might reduce compressibility and/or impede reducing permeability.
Table 5 also presents the experimental results when the soil samples were conditionedwith both foam and polymer slurry. Since the compressibility requirement can be easilymet by adding polymer slurries, the minimum FIR was chosen to be 22%. The PIR ofthe polymer slurry (concentration of 0.3%) was varied from 8% to 30%. The table showsthat the slump value requirement was satisfied when the PIR was either 8% or 15%.However, foam and polymer outflow were observed during the slump test, as can beseen in Figure 9b when the PIR was at a higher value of 15%. The segregation behavioralso reduces the compressibility of the conditioned soil. At PIR of 30%, the foam and thepolymer experienced more serious material segregation during the slump test, and thespecimen collapsed (see Figure 9b). This means that the plastic behavior of excavated soilsis disrupted when excess polymer slurry is used.
In contrast, when the polymer slurry was added at PIR of 8%, the compressibilityincreased; this was because the highly viscous polymer slurry trapped the foam so that it didnot flow out from the soil sample. In other words, the polymer slurry changed the rheologyof the excavated soils, preventing the segregation behavior [16]. Table 5 and Figure 10 showthat the soils conditioned by adding foam at FIR = 22% together with polymer slurry atPIR = 8% or 15% met the required permeability coefficient of 1 × 10−3 cm/s.
Appl. Sci. 2021, 11, 2995 12 of 15
Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 15
(a) (b)
Figure 9. Slump test results depending on added conditioning agents: (a) Foam conditioning only, (b) Foam and polymer
slurry conditioning.
As can be observed from the appearance of the conditioned soil in Figure 9a, foam
and soil did not interact as a fully mixed material. The foam segregated from the soil,
moved upward when tamped with a tamping rod to prepare for the slump tests, and fi-
nally flowed out from the soil samples as shown in Figure 9a. This segregation behavior
between foam and excavated soil might reduce compressibility and/or impede reducing
permeability.
Table 5 also presents the experimental results when the soil samples were condi-
tioned with both foam and polymer slurry. Since the compressibility requirement can be
easily met by adding polymer slurries, the minimum FIR was chosen to be 22%. The PIR
of the polymer slurry (concentration of 0.3%) was varied from 8% to 30%. The table shows
that the slump value requirement was satisfied when the PIR was either 8% or 15%. How-
ever, foam and polymer outflow were observed during the slump test, as can be seen in
Figure 9b when the PIR was at a higher value of 15%. The segregation behavior also re-
duces the compressibility of the conditioned soil. At PIR of 30%, the foam and the polymer
experienced more serious material segregation during the slump test, and the specimen
collapsed (see Figure 9b). This means that the plastic behavior of excavated soils is dis-
rupted when excess polymer slurry is used.
In contrast, when the polymer slurry was added at PIR of 8%, the compressibility
increased; this was because the highly viscous polymer slurry trapped the foam so that it
did not flow out from the soil sample. In other words, the polymer slurry changed the
rheology of the excavated soils, preventing the segregation behavior [16]. Table 5 and Fig-
ure 10 show that the soils conditioned by adding foam at FIR = 22% together with polymer
slurry at PIR = 8% or 15% met the required permeability coefficient of 1 × 10−3 cm/s.
Figure 10. Permeability coefficient changes of the conditioned Soil 5 with elapse of time.
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
0 100 200 300 400 500 600
Per
mea
bil
ity C
oef
fici
ent
(cm
/s)
Time (min)
Un-conditioned FIR 45 %
FIR 22 % + PIR 8 % FIR 22 % + PIR 15 %
1×10-2
1×10-3
1×10-4
1×10-5
1×10-1
Figure 10. Permeability coefficient changes of the conditioned Soil 5 with elapse of time.
In summary, the conditioned soils satisfied all the requirements when conditioned withfoam at 22% and polymer slurry at 8% as long as the water content in the chamber couldbe controlled at around 10%. However, as we mentioned in the introduction, the naturalwater content of the saturated granite soils (below the groundwater table) on the KoreanPeninsula are within the range of 11–21% in most cases [14]. In the laboratory experiments,it was not easy to prepare soil samples for slump tests (even when polymer slurry wasadded along with foam) if the water content was greater than 10%. Since the water contentwas mostly greater than 11%, it might be necessary to add solidification agents along withfoam and polymer slurry for tunnels excavated below the groundwater table.
It is concluded that the EPB shield TBM is feasible even though weathered granitesoil is extremely coarse (cohesionless) and exceeds the applicable boundaries proposedby [1]. However, the process requires conditioning with foam, polymer slurry, and/orwater-absorbing polymers, and it is necessary to test different conditions through trialand error to produce soils that meet the three requirements or workability, compressibility,and permeability requirements.
Finally, a flow chart describing the process of how to choose a suitable conditioningagent is prepared as shown in Figure 11 so that readers can easily follow the processby themselves.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 15
In summary, the conditioned soils satisfied all the requirements when conditioned
with foam at 22% and polymer slurry at 8% as long as the water content in the chamber
could be controlled at around 10%. However, as we mentioned in the introduction, the
natural water content of the saturated granite soils (below the groundwater table) on the
Korean Peninsula are within the range of 11–21% in most cases [14]. In the laboratory
experiments, it was not easy to prepare soil samples for slump tests (even when polymer
slurry was added along with foam) if the water content was greater than 10%. Since the
water content was mostly greater than 11%, it might be necessary to add solidification
agents along with foam and polymer slurry for tunnels excavated below the groundwater
table.
It is concluded that the EPB shield TBM is feasible even though weathered granite
soil is extremely coarse (cohesionless) and exceeds the applicable boundaries proposed by
[1]. However, the process requires conditioning with foam, polymer slurry, and/or water-
absorbing polymers, and it is necessary to test different conditions through trial and error
to produce soils that meet the three requirements or workability, compressibility, and per-
meability requirements.
Finally, a flow chart describing the process of how to choose a suitable conditioning
agent is prepared as shown in Figure 11 so that readers can easily follow the process by
themselves.
Figure 11. Soil conditioning agent selection for EPB shield TBM in weathered granite soil.
4. Application Ranges of EPB Shield TBM in Weathered Granite Soil Ground
Figure 1 shows the application ranges of the EPB shield TBM proposed by [1], com-
posed of three zones. As shown in the figure, soil conditioning with foam is possible in
Zone I; however, in Zones II and III where the particle sizes are larger, that is, the soil is
coarser, it is necessary to add polymers and fines in addition to foam to increase fine par-
ticle content and induce plasticity. By taking advantage of the particle-crushing effect of
weathered granite soil and considering the findings presented here, new application
ranges are proposed that are directly applicable to weathered granite soil. These are
shown in Figure 12; the range proposed by [1] is presented in the same figure for compar-
ison purposes.
First, the ranges are divided into two categories by water content: A and B when the
content is within the allowable range and A′ and B′ when the water content is excessive.
Zone A (yellow area) in Figure 12 represents the region in which only foam is added for
soil conditioning because the water content is within the allowable range (preferably the
optimal value) shown in Figure 5; Zone A′ (yellow area) in Figure 12 represents the region
in which water-absorbing polymer was needed along with the foam because of water con-
tent of excavated soil above the upper bound shown in Figure 5.
Zone B (orange-colored area) represents the region in which polymer slurry is
needed for soil conditioning in addition to foam when the water content is controlled at
Figure 11. Soil conditioning agent selection for EPB shield TBM in weathered granite soil.
Appl. Sci. 2021, 11, 2995 13 of 15
4. Application Ranges of EPB Shield TBM in Weathered Granite Soil Ground
Figure 1 shows the application ranges of the EPB shield TBM proposed by [1], composedof three zones. As shown in the figure, soil conditioning with foam is possible in Zone I;however, in Zones II and III where the particle sizes are larger, that is, the soil is coarser, it isnecessary to add polymers and fines in addition to foam to increase fine particle content andinduce plasticity. By taking advantage of the particle-crushing effect of weathered granitesoil and considering the findings presented here, new application ranges are proposed thatare directly applicable to weathered granite soil. These are shown in Figure 12; the rangeproposed by [1] is presented in the same figure for comparison purposes.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 15
around 10%; Zone B′ (orange-colored area) represents the region in which foam, polymer
slurry, and thickening agent are needed for soil conditioning for digging out tunnels be-
low the groundwater table (water contents larger than 10%).
Figure 12. Application ranges of EPB shield TBM in weathered granite soil ground.
The application ranges proposed in Figure 12 can be used even if the water content
of the excavated soil is less than the lower bound shown in Figure 5 in Zone A; however,
it is important to add water to the chamber so that water content can reach the allowable
range and preferably the optimum value shown on curve A in Figure 5. Similarly, in Zone
B, if the ground is only partially saturated, water must be added to reach 10%.
Figure 12 clearly shows that both zones expand more toward the right (coarser soil)
than do the zones proposed by [1]. The main reason for the expanded application range
could be the particle-crushing effect of the weathered granite soil, which moves the parti-
cle-size gradation curve of the excavated soil to the left after soil conditioning.
5. Conclusions
Soil conditioning methods applicable to the EPB shield TBM when the ground is
composed of weathered granite soils were studied in this paper; the properties of the con-
ditioned soils, i.e., workability (slump value), permeability, and compressibility, were
evaluated in laboratory scale to find the optimum conditioning methods depending on
soil water content. Application ranges of the EPB shield TBM in weathered granite soil
were then proposed. The study conclusions are as follows.
Controlling water content to within the allowable range (see Figure 5) is the most
important factor for successful EPB shield TBM operation. Optimal (or at least allowable)
water content increased with increasing fine particle content in the soil.
It was found that if the water content of excavated soil is well controlled within the
allowable range (see Figure 5) and the fine particle content finer than #200 sieve size is
more than 1.2%, soil can be conditioned with foam only with an optimal FIR range of 22–
67%. However, if water content exceeds the upper bound, water-absorbing polymer (so-
lidification agent) should be added in addition to foam to change fluidic soils to plastic.
Figure 12. Application ranges of EPB shield TBM in weathered granite soil ground.
First, the ranges are divided into two categories by water content: A and B when thecontent is within the allowable range and A′ and B′ when the water content is excessive.Zone A (yellow area) in Figure 12 represents the region in which only foam is added forsoil conditioning because the water content is within the allowable range (preferably theoptimal value) shown in Figure 5; Zone A′ (yellow area) in Figure 12 represents the regionin which water-absorbing polymer was needed along with the foam because of watercontent of excavated soil above the upper bound shown in Figure 5.
Zone B (orange-colored area) represents the region in which polymer slurry is neededfor soil conditioning in addition to foam when the water content is controlled at around10%; Zone B′ (orange-colored area) represents the region in which foam, polymer slurry,and thickening agent are needed for soil conditioning for digging out tunnels below thegroundwater table (water contents larger than 10%).
The application ranges proposed in Figure 12 can be used even if the water contentof the excavated soil is less than the lower bound shown in Figure 5 in Zone A; however,it is important to add water to the chamber so that water content can reach the allowablerange and preferably the optimum value shown on curve A in Figure 5. Similarly, in ZoneB, if the ground is only partially saturated, water must be added to reach 10%.
Appl. Sci. 2021, 11, 2995 14 of 15
Figure 12 clearly shows that both zones expand more toward the right (coarser soil)than do the zones proposed by [1]. The main reason for the expanded application rangecould be the particle-crushing effect of the weathered granite soil, which moves the particle-size gradation curve of the excavated soil to the left after soil conditioning.
5. Conclusions
Soil conditioning methods applicable to the EPB shield TBM when the ground iscomposed of weathered granite soils were studied in this paper; the properties of the con-ditioned soils, i.e., workability (slump value), permeability, and compressibility, were eval-uated in laboratory scale to find the optimum conditioning methods depending on soilwater content. Application ranges of the EPB shield TBM in weathered granite soil werethen proposed. The study conclusions are as follows.
Controlling water content to within the allowable range (see Figure 5) is the mostimportant factor for successful EPB shield TBM operation. Optimal (or at least allowable)water content increased with increasing fine particle content in the soil.
It was found that if the water content of excavated soil is well controlled within theallowable range (see Figure 5) and the fine particle content finer than #200 sieve size ismore than 1.2%, soil can be conditioned with foam only with an optimal FIR range of22–67%. However, if water content exceeds the upper bound, water-absorbing polymer(solidification agent) should be added in addition to foam to change fluidic soils to plastic.
Even with extremely coarse weathered granite soil (fine particle content less than1.2%), the EPB shield TBM can still be adopted; however, polymer slurry (and additionalwater-absorbing polymer when digging out tunnels below the groundwater table) shouldbe added with foam to change the cohesionless state of the soil to sticky or plastic.
Application ranges of the EPB shield TBM of the weathered granite soil proposed inthis study revealed that ranges were more expanded to the right (coarser) side comparedwith those proposed by [1]. The main reason for the expanded application range mightbe the particle-crushing effect of the weathered granite soil, which moves the particle-sizegradation curve of the excavated soil to the left after soil conditioning.
Author Contributions: Conceptualization, T.-H.K. and I.-M.L.; Formal analysis, T.-H.K.; Funding ac-quisition, I.-M.L. and J.-J.P.; Investigation, T.-H.K., H.-Y.C. and Y.-M.R.; Methodology, T.-H.K.; Projectadministration, T.-H.K. and I.-M.L.; Writing—original draft, T.-H.K.; Writing—review and editing,I.-M.L. and Y.-M.R. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the National Research Foundation of Korea (NRF) grantfunded by the Ministry of Science and ICT of Korean government (MIST) (No. 2019R1C1C1008326)and Infrastructure and Transportation Technology Promotion Research Program funded by the Min-istry of Land, Infrastructure and Transport of Korean government (18SCIP-B066321-06, Developmentof Key Subsea Tunneling Technology).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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